Cable Fault Locating Fault Profiles Guide
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TDR LEGEND Open Conductor/Neutral
Shorted Conductor/Neutral
Splice
Wet Splice
High Resistance Splice
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Transformer
Tee or Wye
Water Ingress
Cable Fault Locating Fault Profiles Guide
OPEN CONDUCTOR (blow-out)
Circuit Equivalent
Open conductor or blow-out occurs due to a high-fault current, a dig-in or a failed splice. An insulation resistance test may indicate a high resistance value if the fault and ground is dry. If the ground is moist, the resistance measurement may show a moderate to low resistance value at the fault. If the phase conductor or concentric burns back, the gap distance may be excessive and may not arc or thump. (Refer to fault position A.) Testing from the other end of the cable may render better results as shown in position B. Example of a typical open conductor trace.
Surge Generator
Voltmeter
During a surge discharge (thump), the voltmeter will give a weak drop and will never approach zero volts (fault A). Fault B will give a strong drop in the voltmeter, practically reaching zero.
An open conductor (or blow-out) will appear as a positive reflection on a time domain reflectometer (TDR) as shown by the solid TDR signature. If the conductor or concentric is burned back and the gap value is excessive, the fault may not arc (thump). This will result in little or no change in the TDR signature when the arc reflection test is performed. The solid signature shown above represents the before surge TDR signature; the dotted line represents the after surge signature.
Ammeter
During a surge discharge As stated previously, an open conductor (or blow-out) will (thump), the ammeter will give a appear as a positive reflection on a TDR. If the gap distance weak kick forward (fault A). Fault between the conductor and concentric has not burned back, B will give a strong kick forward. the fault image will invert or reflect negative when the arc reflection test is performed as seen in the dotted TDR trace. This also indicates good arc or thump condition.
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Cable Fault Locating Fault Profiles Guide
SHORTED CONDUCTOR (bolted fault)
A shorted conductor (or bolted fault) occurs when the insulation burns to a state where the cable conductor and cable neutral (or sheath) is in contact with one another. Insulation resistance is zero to only a few ohms. There is no gap value and therefore this fault will not arc (or thump). Circuit Equivalent
Example of a typical shorted conductor low voltage trace.
Surge Generator
Voltmeter
During a proof test the kilovoltmeter will read zero and will show no movement
Ammeter
During a proof test the ammeter will continue to rise as the output of the proof tester is raised.
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A shorted conductor (or bolted faults) can be localized by using time domain reflectometry (TDR). The reflected pulse will be negative. Since there is no gap value associated with a shorted conductor, the fault will not arc (or thump). Acoustic pinpointing will not be an option. A surge generator (thumper) can still be used for pinpointing if an electromagnetic impulse detector is used along with the surge generator. Surge the cable with the surge generator. Track the surge pulse using an electromagnetic detector such as the Megger MPP2000 Pinpointer. You will detect strong signals up to the true fault location. Since the entire surge energy is lost at the fault, weak or no signal will be detected beyond the fault.
Cable Fault Locating Fault Profiles Guide
ELECTRICAL TREE (pinhole)
Circuit Equivalent
Electric trees (or pinholes) are the most common fault found in underground electrical power cable. The fault starts through a process of water ingress or small defects in the dielectric. Through age, these defects begin to grow, taking on the pattern of tree branches. Eventually, one of these branches will breach the cable insulation and become a fault. Example of a typical pinhole trace.
Surge Generator
Voltmeter
After discharge from the surge generator (thumper) the voltmeter will give a strong kick back, nearly reaching zero volts.
The time domain reflectometer signature will first show the end of the cable under test. The cable end will be seen as a positive reflection indicating an open. Refer to the signature shown with a solid line. After the arc reflection test, a negative reflection will be captured from the point of the fault. The fault will be shown as a negative reflection indicating the short circuit created by the arc (thump).
Ammeter
After discharge from the surge generator (thumper) the ammeter will give a strong kick forward.
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Cable Fault Locating Fault Profiles Guide
SPLICE FAILURE
A failed splice can be a difficult fault to identify and locate. Since the physical distance between the phase conductor and concentric is increased, the gap distance is typically excessive. This creates a fault that may not arc or thump. Example of a typical splice failure trace. Circuit Equivalent
Surge Generator
Voltmeter
During a surge discharge (thump), the voltmeter will give a weak drop and will never approach zero volts. The fault may not arc or break down on every discharge cycle. During a proof test, the cable may hold voltage for a brief period before flashing over.
Ammeter
During a surge discharge (thump), the ammeter will give a weak kick forward. Burn current may be necessary to reduce the fault resistance and gap distance. www.megger.com/us
A splice failure is very similar to an open conductor (blow-out) in the sense that the resistance and gap value at the fault will typically be very high. The excessive gap value may make it difficult to arc or thump the cable. The fault may not arc on every discharge cycle. Applying burn down current may be required to improve breakdown performance.
The time domain reflectometer pattern for a splice is a small positive reflection that is immediately followed by a small negative reflection. (Refer to the solid signature shown above.) If water is present within the splice, the negative reflection will typically be more pronounced. When the arc reflection test is performed, the signature of the splice may change slightly, becoming more pronounced on the negative (as shown in the dotted signature above.) This indicates that during the surge (or thump) the resistance of the splice is becoming lower and that a thump is occurring. If the gap between the conductor and concentric is too excessive, the arc reflection test will show no change in the TDR signature. This indicates that no thump is occurring and burn-down may be required.
Cable Fault Locating Fault Profiles Guide
CORRODED OR OPEN CONCENTRIC
Circuit Equivalent
Surge Generator
WARNING: Corroded or open concentric will present several issues during a fault locate. The concentric is designed to carry fault current. If the integrity of the concentric is jeopardized due to open strands or high levels of corrosion, the concentric may no longer provide the lowest resistance path back to the surge generator. In this case, the fault current will follow the least-resistant path, which may be fence posts, phone shields, cable TV shields, gas tracer wires, other utility cables or the damp earth itself. Humans or animals touching the fence post, phone, TV, waterline, etc., can be injured or killed. Use of a surge generator (thumper) should be approached with great caution. Surge at the lowest possible voltage and for the shortest amount of time. Example of a corroded or open concentric trace.
Voltmeter
The meters on the surge generator (thumper) will provide little to no indication of concentric corrosion. If an arcing fault exists on the cable, the meters will simply show a sharp drop in voltage and a sharp kick in current when the cable is surged (thumped).
Ammeter
The time domain reflectometer signature can help identify possible corroded concentric issues prior to using a surge generator or thumper. Small random reflections, typically positive in orientation, may be observed indicating areas of high-metallic resistance. It may be difficult to detect the end of the cable due to excessive attenuation of the TDR’s transmitted pulse. When the arc reflection test is performed, a negative reflection will be captured at the location of the arc or thump. Safety precautions should be taken during the pinpointing stage. Corroded concentric presents step-potential hazards.
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MEG-762/MIL/1M/V2/6.2012
